In recent years, a diesel particulate filter (DPF) which traps particulates discharged from a diesel engine has attract attention, and there has been a demand for a DPF capable of obtaining a high trapping efficiency with a low pressure loss. As the DPF, a honeycomb structure formed of cordierite has heretofore been used, and a porosity, pore distribution and the like of the honeycomb structure have heretofore been improved in order to obtain the above-described high trapping efficiency with the low pressure loss.
As conventional techniques, a honeycomb structure whose porosity and average pore diameter are increased and whose pore distribution in a partition wall surface is defined, and a honeycomb structure whose partition wall thickness is set to be not more than a predetermined thickness and whose porosity is increased have been described (see Patent Documents 1, 2).
Moreover, as another conventional technique, manufacturing of a high-porosity honeycomb structure by use of particles coarser than predetermined as particles of both talc and silica components in a cordierite forming material, and a honeycomb structure whose porosity is increased and whose pore distribution and surface roughness are defined have been described (see Patent Documents 3, 4). It is to be noted that in these conventional techniques, to raise the porosity, the cordierite forming material is formed into coarse and large particles, and graphite, wood powder, foaming agent or the like is added as a pore former.
However, when the cordierite forming material is formed into the coarse and large particles, a cordierite forming reaction does not sufficiently proceed, and it has been difficult to achieve a low coefficient of thermal expansion. In a case where graphite is used as the pore former, permittivity of a formed body to which graphite has been added drops. When the added amount increases, it is difficult to uniformly perform drying by dielectric drying or microwave drying. Further in a firing step, a firing time is lengthened at 800 to 1000° C., and therefore there has been a problem that rapid burning of graphite needs to be inhibited.
Moreover, when starches and wood powders are used as the pore formers, a large amount of water needs to be added in order to form a clay into a predetermined hardness in a kneading step, and there has been a problem that an efficiency of a drying step is degraded. Further in a firing step, the starches and wood powders cause rapid burning and generate heat at 200 to 400° C., and it has been difficult to prevent firing cracks from being caused by this step. In this manner, in the conventional techniques, it has been remarkably difficult to increase an open porosity to be not less than a predetermined porosity.
Moreover, to obtain a higher open porosity, for example, an open porosity of 60% or more, a large amount of pore formers need to be also added. When a large amount of organic compound based pore formers are added in this manner, amounts of an organic volatile substance, and a gas such as carbon dioxide generated in a degreasing (calcining) stage also increase, and a burning heat also increases. Therefore, defective portions such as cracks, tears and cuts, that is, defective places which do not exert any filter function and in which leakage of a fluid occurs are formed in an obtained calcined (degreased) body or fired body in some case.
(Patent Document 1)
Japanese Patent Application Laid-Open No. 9-77573
(Patent Document 2)
Japanese Patent Application Laid-Open No. 11-333293
(Patent Document 3)
Japanese Patent Publication No. 7-38930
(Patent Document 4)
Japanese Patent No. 2726616
The present invention has been developed in consideration of these problems of the conventional techniques, and an object thereof is to provide a method of manufacturing a cordierite porous body having a high open porosity, in which there is an extremely low possibility of generating defects causing leakage of a fluid, such as cuts.